Production of potassium silicofluoride



June 5, 1951 P. CALDWELL ET AL Filed May `22, 1946 PRODUCTION 0FPOTASSIUM SILICOFK'LUORIDE WATER FLOW METER LUT/NG TANK Ne /EK lNVENTORSPAUL CALDWEZZ D4! E' CLA P/(E K/ff/rf'R ATTORNEYS Patented June 5,14951l PRODUCTION F POTASSIUM SILICOFLUORIDE Paul Caldwell, Portlock,and Dale Clarke Kieffer, Norfolk, Va., assignors to SmithDoug1ass-Company, Inc., Norfolk, Va., a corporation of Virginia Application May 22,1946, Serial N0. 671,440

Claims. (Cl. 23-88) This invention relates to the production ofpotassium silicoiiuoride.

The reaction of Water with silicon fluoride (Silla) yields iiuosilicicacid liquor, sometimes called fluorne liquor, which is a solution offluosilicic acid (I-IzSiFc) containing silica in suspension. Fluorineliquor is commonly produced in connection with superphosphatemanufacture, by absorbing in water gases generated in superphosphatedens. When a saturated solution or brine of commercial sodium chlorideis added to fluorine liquor, a reaction yielding sodium silicofluoride(NazSiFa) and hydrochloric acid occurs. The sodium silicouoride developsas crystals which settle to the bottom of their mother liquor. Byappropriate separation, such as decanting the supernatant liquor,centrifuging the residual slurry and washing and drying the crystals,commercial sodium silicofluoride may be readily obtained.

In attempts to apply this known procedure for the production ofpotassium silicouoride (KzSiFs) by adding to the iluosilicic acid liquora brine of potassium salt, such as commercial potassium chloride,instead of sodium chloride, a reaction yielding potassium silicoiiuorideoccurs, but the potassium silicouoride develops in a highly dispersedstate as a substantially uniilterable or colloidal gel that is difhcultif not impossible to separate from the motor liquor. Though thisgelatinous form of potassium silicoluoride be separated and driedpainstakingly, it is difficult to handle or use, and it is evenunsuitable for certain industrial usesV that require briquettedmaterial, because briquettes made of it disintegrate when subjected toroasting treatment.

The principal object of this invention is to provide a process wherebythe reaction of fluosilicic acid with potassium chloride to producepotassium slicofluoride is made to yield the potassium silicofluoride ina distinctly crystalline form, which enables the product to be isolatedreadily from its mother liquor and makes possible the commercialproduction of potassium silicofluoride in a controlled and economicalmanner.

Another object is to provide a process for enhancing the size offormation, and hence the lterability, of the potassiumsilicofluoridecrystals.

Another object of the invention is to provide an improved crystallineform of commercial potassium silicofluoride that can be compressed intobriquettes which retain their shape under rough handling conditions andwhich withstand heat or roasting treatment Without disintegrating, sothat a product serving important industrial needs is obtained. l

A further object of the invention is to provide a cyclical process forthe commercial production of crystalline potassium silicofluoride, bywhich economies of process materials and heat may be achieved. And yetanother object is to adapt said process for practical use in acontinuous manner, as well as by batch operations.

The present invention is based upon the discovery that the developmentof potassium silicoriuoride in a gelatinous state when following theprocedure above mentioned is attributable to a dispersing or peptizingeffect of concentrated solutions, or brines, made of commercialpotassium salts such as potassium chloride, and the invention utilizesthe further discovery that the potassium silicofluoride will develop andcan be isolated easily in a distinctly crystalline, nongelatinous formif to a suitable aqueous solution or liquor of iiuosilicic acid there isadded a dilute aqueous solution of potassium chloride or other suitablesalt that is substantially free of the insoluble impurities normallypresent in the commercal form of said salt.

When dilute solutions of commercial potassium salts are used for thereaction Without any previous purification the gelatinous form ofpotassium silicoiluoride still occurs, and it occurs also when potassiumsalt brines that have been filtered to remove suspended impurities areused in concentrated or substantially saturated form. The desired easilylterable, crystalline form of potassium silieorl'uoride can be obtained,however,` by the use of dilute solutions of commercial potassium saltsfrom which insoluble matter previously has'been removed by filtration orthe like.

A crystalline product easy to separate from the mother liquor and easyto dry can be obtained, for example, by reacting the uosilicic acidsolution or liquor with a ltered aqueous solution of commercialpotassium chloride containing about 30 grams of the salt per liter. Onthe other hand, a gelatinous product results when the solution containsabout 40 grams of potassium chloride per liter or a higherconcentration. In general, satisfactory results can be achieved bykeeping the concentration of the potassium chloride solution within amaximum limit of about 35 grams per liter, and concentra tions of about30 grams per liter, more or less, preferably are used for the bestresults. Although still lower concentrations, or greater dilutions, alsoyield easily lterable crystals, they may add unnecessarily to the volumeof liquids to process and to the amount of potassium silicofluoride lostin solution in the mother liquor- It has been discovered, further, thattemperatures exert an important inuence upon the formation ofcrystalline potassium silicouoride by the process above disclosed, andthat increasing the temperature of the reacting solutions above acertain minimum affords a means of enhancing the size of the crystalsthat form. Diiiiculty in separating and drying the prorl uct generallyresults if the temperature of the reacting solutions is below about 70F. At higher temperatures, however, potassium Y silicorluoride crystalsof readily lterable size are formed, which appear under a microscope tobe entirely free of the presence of gel. carrying out the reaction atabout 95 F. or above, crystals several times larger than those yieldedlat 70 F. may be obtained.

The principles above described may be utilized, according to preferredembodiments of the invention, by making afconcentrated solution or brineof commercial potassium chloride, l

.tering it, then diluting it to a concentration of about 30 grams ofpotassium chloride, more or less7 lper liter with aqueous liquid .thatis substantially free. of insoluble impurities, heating the dilutesolution to about 115 F., and then adding it to a solution or liquor ofi'luosilicio acid. at'about room temperature, say 70 F. Highertemperatures below boiling can be used with like effect, though atanunnecessary expense for heat. The liquids are agitated while being addedtogether, and fllterable potassium silicouoride crystals immediatelyform which require only a few minutes to settle to the. bottom of the,mother liquor.` They may be isolated by dhltration or, preferably, bythe procedure used in producing Y sodium silicofluoride, i. e., allowingthe crystals to settle, decanting the supernatant mother liqu ucr,centriiuging the residual slurry, washing and partially drying thecrystals in the centrifuge, 'and then drying them completely in anysuitable manner. A commercially pure, dry, crystalline form of'potassium silicoiluoride re sults, .the average analysis of which isabout 98.0 %r potassium silicofluoride.

It has been found advantageous to use a sub,n stantially clearfluosilicic solution or iiuorine liquor, rather than the cloudy fluorineliquor containing suspended silica that results directly from thereaction of. silicon fluoride with water. A clear liquor. is obtained byfiltering out or otherwise removing the silica. Although the cloudyuorine liquor yields a crystalline prod uct that settles well with agood layer separa tion, the small amount of adsorbed silica contaminatesthe product and raises the water conftent ofthe centrifuged product.

The influences of dilution, temperature and suspended impurities uponthe results achieved pursuant hereto are somewhat relative. For example,the use of a highly dilute solution of potassium salt, Vsuch as onecontaining substantially less than 30 grams of KCl per liter, will pernmit the useof relatively lower temperatures or the presence ofrelatively larger amounts of in solubleimpurties than otherwise wouldpermit the attainment of the required results. Sirn-- ilarly, the use ofrelatively high reaction ternperatures ranging upwardy from 95 FV. willpermit the use of relatively more concentrated solutions f, POWSU??5h11?, 0.1.". falle.. peem?? 0f. 1mg?? 4 amounts of insolubleimpurities, than otherwise would be feasible.

It has been found, further, that the production of substantially pure,crystalline potassium silicoiiuoride according to the foregoingprinciples can be carried out satisfactorily in continuing commercialoperations by utilizing some of the mother liquor separated from thereaction product in one batch or phase of the operations, together withan added amount of clear Water, in preparing the dilute solution ofpotassium salt to be used for a succeeding batch or phase of theoperations. The dilute solution so prcpared by recyclingA mother liquoryields a satisfactory crystallineproduct that iilters and dries well,even though it contain substantial concentrations of hydrochloric orother acid and other soluble impurities' that tend to reduce crystalsize. Y

For the best results, not more than of the volume of the dilute solutionshould be. composed of recycled mother liquor, and 'the Icycling shouldnot bc continued so loue. that .impurities accumulate to an intolerableextent. A satisfactory practice is to re-start the cyclical process cachday by making. and usine a iro-sii solution of lltcrcol potassium .saltbrine diluted outrcly with clear Water. The. recycling piracticc,properly controlled. gives advantages under many practical conditions,in that a larger amount of the uuroactcd excess ol potassium salt may bcrecovered by the Vaction of saltug out from the discarded mclhcr liquor,@IHOUD 0f DQE'/ lglllel @0., lnpi? th@ desired temperature to the.Vdilute solution is reduces, aud. the amounts of. Clearwater required theDYQQQS. 3.1"@ llledu The DQllSSlllll S3411? gllr' orally is used sexcess. and slncc. it is sx.- psusivo thc advautaaa moutoucd, involves aconsiderable saving.

While. we have. referred, mainly. to the use of potassium chlorideY inthe practccof our iuvcn.- tion. because it is. the potassium salt most.available aud most lilscly to. bc. employed. substantallvauy of bloc.many watcrrsolublo acid and normal. salts of potassium also. bc used ac-@Ording to thc samoprip plus.; for example., po tassium sulfate. potassim. nitrato. potassium bromideJ `potassulou iodide, potassium acid car.-bonatc. potassium acidsulfatc, etc. 'Ihe dilute solutions of. these.other salts. should be mada to contain thcv same. quantities of. potashpor liter, calculated as potassium, as. arc.. cffoctivc in. the

usc of dilute solutions of potassium chloride. un-

derv comparableY conditions. This is. to say4 that the. dilutepotassium' salt solution usually should contain not more than about 19grams of Ipotash, calculated as. potassium, por, 'liteitY and that aconcentration. of. about 1 6' grams of` potassium pcrltcl. more or less.represents the. practice presently preferred, subject. oi course totvaria.- tions according to the. size o f,` crystalfcrmation desired andthe prevailing adjustment. of; the

, rolativoiniiuences described, above..

grams of iiuosilicic acid per liter are delivered from a measuring tankl, whileat room temperature, into a reaction tank 2 equipped with anagitator 3, and the agitator is started. One thousand and fifty-rive(1055) gallons of a dilute solution of ltered potassium chloride brine,prepared as described below to contain about 30 grams of KCl per liter,and heated to 115 F., are added through line 4 to the clear liquor inthe reaction tank, while agitating. When all the dilute solution hasbeen added, the batch is allowed to settle ve minutes. Then thesupernatant mother liquor is siphoned or decanted from the reaction tankinto measuringtank 5, from which 636 gallons of the mother liquor issent to a diluting tank 6, while the remainder of the supernatant motherliquor is discarded to waste. The residual slurry in the reaction tankis discharged to a centrifuge in which the crystals are purged, washedwith clear water, and spun to a nearly dry state. The centrifugedcrystals then are passed through a rotary drum drier, from which theyissue as dry, commercially pure, crystalline potassium silicoiiuoride.

At the start of each days operations, the dilute solution of potassiumchloride is prepared by combining about 181 gallons of filteredsaturated -potassium chloride brine (containing about 300 grams of KClper liter), as obtained from measuring tank 1, in the diluting tank 6with enough clear water, admitted at 8, to make about 1055 gallons ofdilute solution that contains about 30 grams of KCl per liter. Thisdilute solution is heated to 115 F. and used for the iirst batchreaction, as above described. The 636 gallons of mother liquor removedfrom the batch through tank 5 to the diluting tank 6 is then used tomake a further batch of dilute KCl solution by adding about 181 gallonsof saturated brine from tank 'l as aforesaid and also enough clear waterat 8 to obtain another 1055 gallons of dilute solution, which in turn isheated to 115 F. and used for the next batch reaction. The recycling ofmother liquor decanted from one batch as part of the dilute solutionused for the next batch reaction is continued until the end of the daysoperations, when all of the mother liquor decanted from the last batchis sent to waste.

In the continuous operations shown diagrammatically in Fig. 2, theconcentrations and proportions of the reagents used may be the same asin the example of batch operations. At the start, a quantity of dilutesolution having the desired concentration of potassium salt is preparedand heated to the desired temperature in diluting tank 6a. Clear1fiuorine liquor is fed at a regulated rate through a iiow meter I a intoreaction tank 2a where agitator 3a is working, while the dilute solutionis fed from tank 6a into the reaction tank 2a through a iiow meter 4a,at a rate regulated so as to furnish enough potash in solution forreaction with all the luosilicic acid in the liquor, and some excess.These reagents are introduced at a distance sufficiently below theoverliow line 5a, by which mother liquor is decanted, to assure thatthey will react completely and that the potassium silicofluoridecrystals formed will settle from the mother liquor before it reaches theoveriiow. As the reaction and the inflow of reagents proceed in tank 2a,all the excess mother liquor is drawn or decanted from that tank throughline 5a, and a proportion of the mother liquor, as desired for therecycling practice, is passed through ow meter 5b into the diluting tank6a, the remainder being sent `to Waste.

Vfor separation of the crystals, after which the crystals may be driedcompletely in any desired 'manner `The crystalline potassiumsilicouoride produced according to our invention can be made intobriquettes by usual briquetting procedures, and the briquettes soobtained possess the valuable quality of withstanding heat and roastingtreatment without disintegrating, whereas briquettes made of thegelatinous potassium silicofluoride heretofore produced ily apart in theovens when subjected to the same roasting conditions.

Various modifications and substitutions of the details and examples setforth hereinabove will become apparent to those skilled in the art andmay be resorted to without departing from the invention, which isintended to be deiined by the appended claims.

We claim:

l. A process for producing potassium silicoiiuoride which comprisesreacting at a temperature not substantially below F. an aqueous solutionof iiuosilicic acid substantially free of silica with a dilute aqueoussolution oi potassium chloride containing not more than about 35 gramsof potassium chloride per liter and substantially free of the insolubleimpurities normally present in commercial potassium chloride, so thatpotassium silicouoride crystals readily separable from their motherliquor are formed.

2. A process for producing potassium silicoiiuoride which comprisesproviding at about room temperature a solution of fluosilicic acidconstituting a reaction product of silicon fluoride and water from whichsilica has been removed, providing a dilute solution of potassiumchloride containing not more than about 35 grams of potassium chlorideper liter, heated to at least about F. and substantially free of theinsoluble impurities normally present in commercial potassium chloride,adding to said fluosilicic acid solution, while agitating it, enough ofsaid dilute solution to react with all the fluosilicic acid present, sothat potassium silicoluoride crystals readily separable from theirmother liquor are formed, and isolating said crystals.

3. A cyclical process for producing potassium silicofluoride whichcomprises adding to a substantially silica-free fluosilcic acid liquor adilute aqueous solution of potassium chloride containing not more thanabout 35 grams of potassium chloride per liter and heated to at leastabout 115 F. and substantially free of the insoluble impurities normallypresent in commercial potassium chloride, so that potassiumsilicofluoride crystals readily separable from their mother liquor areformed, separating the crystals from the mother liquor, and continuingoperations as aforesaid with the use of a dilute solution as aforesaidin which the diluent is composed partly of mother liquor from which thecrystals previously were separated and partly of clear water.

iastate@ ride per liter, b'eIg--Ieated to at nleast about 100 F. andbeing substantially Vfree ef the solllble impurities -hormally presentih ciiiiiercial potassium chloride, so that potassium silicofluoridecrystals readily separable from their mother liquor are formed,'separating the crystals i from the rrioth'er liquor, aid 'repeating thersaid operations successively upon 'successive batches of liquor asaferesad With the Ause et uccssv'e batches of dilutesolutioh asaforesaid iin 'each of which the dilut is combosed partly 'f motherliquoi` from which the crystals previously were separated and 'Iiartlyof clear v\'f.`rater.

V5. A process for producing potassium silicouoride whichV comprisescontinuously iht'od'ucing into a reaction zone having an overflow pointregulated streams of I aqu'ecus solution of fluosilicic acidsubstantially free of sili'c'a and a clear dilute aqueous solution "ofpotassium chloride containing hot more than about 35 grams of KCl perliter and heated to at least about 115 F.,

mixing said streams at a level below said overflow point so that theyreact at such level to yield potassium silicouoride crystals that settleto the bottom of said zone, continuously 8 lailtllg 'excess Vnuit-herliquor from said zone yat isa-iii yerfloxy point, an'd 'removing aislurry of vlfzreta'ssium silicofiuoiide 'crystals from the bottom DALECLARKE KIEFFER.

REFERENCES CITED The Afoll'owiiig references are of record in the flle'of this patent:

Y lIfI-1\1I'I""1D S'S PA'IS FOREIGN PATENTS Number Couhtry Date 222,838Great Britain June Y25, 1925 271,816 Great Britain 1- v Nov. 3, 1927OTHER REFERENCES y Condensed Chemical Dictionary, 1st.A ed., 1919.,'lumen M'Iwhe Chem. Catalog Co., N. Y., pages 3,85 and 388. n

YA Gomprehehsiye 'Ifvreatiseu on Inorganic Chemistry, by Jf. W. Mellor,1925 ed., vol 6, page 9 47. Lon-gmans, Green & Co., N. Y. Y HackhsChemical Dictionary, 2nd. ed., page 745.- -P. Blakstohs Son & Co.,Philadelphia.

1. A PROCESS FOR PRODUCING POTASSIUM SILICOFLUORIDE WHICH COMPRISESREACTING AT A TEMPERATURE NOT SUBSTANTIALLY BELOW 95* F. AN AQUEOUSSOLUTION OF FLUOSILICIC ACID SUBSTANTIALLY FREE OF SILICA WITH A DILUTEAQUEOUS SOLUTION OF POTASSIUM CHLORIDE CONTAINING NOT MORE THAN ABOUT 35GRAMS OF POTASSIUM CHLROIDE PER LITER AND SUBSTANTIALLY FREE OF THEINSOLUBLE IMPURITIES NORMALLY PRESENT IN COMMERCIAL POTASSIUM CHLORIDE,SO THAT POTASSIUM SILICOFLUORIDE CRYSTALS READILY SEPARABLE FROM THEIRMOTHER LIQUOR ARE FORMED.